1. Field of the Invention
The invention relates to a method for measuring the oxygen content in gas mixtures, especially combustion gases, with an electrochemical measuring device which has an oxygen ion-conducting solid electrolyte and two electrodes to which a supply voltage is fed, as well as a circuit for carrying out the method.
2. Description of the Prior Art
Such a method is used preferably for determining the free oxygen content in exhaust gases of combustion plants burning oil, gas and coal.
In determining these oxygen measurements, a measuring device with a measuring probe which has at least one solid electrolyte of the zirconium oxide type and two electrodes, to which a d-c voltage is applied is preferably used. A measuring device of this type is disclosed in German Published Non-Prosecuted Application DE-OS No. 30 38 429, the first electrode of which, serving as the cathode, is acted on by the gas stream to be measured. Thereby, the free oxygen content is extracted electrolytically from the gas stream and is transported in the form of oxygen ions through the solid electrolyte to the second electrode and recombined there to form oxygen molecules. A current signal between the electrodes is formed due to the oxygen ions traveling through the solid electrolyte. This current signal is independent of the temperature if the operating temperature in this measuring method is kept above a critical value of 650.degree. C. During the measurement, the quantity of the sample gas must be kept constant. This can be achieved in the simplest manner by means of a critical nozzle. If these conditions are observed, the direct proportionality between the probe current and the oxygen concentration is maintained.
A disadvantage of this measuring method is that, depending on the magnitude of the probe voltage applied, in addition to the molecular oxygen which is present, chemically bound oxygen is also liberated by electrolytic dissociation of oxygen-containing compounds such as water, carbon dioxide and nitrogen oxides of the gas sample. This electrolytic dissociation of oxygen-containing compounds causes a falsification of the measurement result proper, and a higher oxygen content is indicated in the gas to be measured than is actually present.